1. Field of the Invention
The invention relates to a power supply device comprising a series arrangement of an inductance, a switching device clocked by a control circuit, and a current measuring impedance, which series arrangement is arranged between two terminals to which a supply voltage can be applied, an input resistor via which a voltage appearing across the current measuring impedance can be applied to an input of the control circuit to control the switching device, and a capacitance in parallel with the input of the control circuit.
2. Description of the Related Art
From the English-language abstract of the Japanese document 63-305755(A) of 13.12.1988 a control circuit for a switched-mode power supply is known, which comprises a driver circuit for a pulse-width control signal from the control circuit, a transformer, a power MOSFET and a control signal generation circuit which generates a control signal. The control circuit comprises a comparator, a quasi-trapezoidal wave generator and a reference source. The quasi-trapezoidal wave generator receives a reference voltage and generates a quasi-trapezoidal signal, which is used as a reference signal. The circuit arrangement shown has a resistor arranged between the power MOSFET and ground. A diode, whose cathode is grounded via a capacitor, has its anode connected to the node between the power MOSFET and the resistor. Moreover, the cathode of the diode is connected to ground via a further resistor and is connected directly to an input of the comparator. The resistor, the capacitor and the diode constitute a control signal generation circuit which derives a direct voltage from the voltage produced across the first-mentioned resistor by the current through the power MOSFET, which direct voltage is applied to the comparator as a control signal. In addition, the quasi-trapezoidal wave generator applies a trapezoidal signal to the comparator. The comparator supplies a pulse-width control signal. Accordingly, the pulse width of the pulse-width control signal decreases drastically if the control signal from the control circuit for the switched-mode power supply exceeds the peak level of the quasi-trapezoidal wave. This is in order to achieve a suitable slope of the output current characteristic.
The English-language abstract of the Japanese document 61-293165 (A) of 23.12.1986 discloses an overcurrent protection circuit for a switching regulator. This circuit includes a smoothing capacitor connected to an alternating voltage source. The output signal of this smoothing capacitor is applied to the series arrangement of the primary winding of a transformer and a switching transistor, as a result of which the desired output signal is available across the secondary winding of the transformer. A current detection resistor is arranged in series with the transistor. The detected output signal of the current detection resistor is applied to a further transistor for overcurrent protection. This signal is applied via a network comprising a series arrangement of an inductance and a resistor and, in parallel with the input of the further transistor the parallel arrangement of a further resistor and a capacitance. This circuit arrangement, particularly said series inductance, serves to reduce high-frequency interference in the detected current from the current detection resistor so as to preclude incorrect operation.
Thus, in the known circuit arrangements interference from the current measuring impedance reaches the control circuit via networks which either transmit only direct voltages or low-frequency voltage components but which attenuate or suppress high-frequency components. As a result, it is also possible to attenuate very brief parasitic voltage transients. However, in the event of a short-circuit at the output of the power supply device, i.e. particularly in the case of a short-circuit of the secondary winding, the high attenuation causes the maximum output current to become substantially larger than in the case of dimensioning for a low attenuation, because the time during which the switching transistor is in its on-state is increased with increasing attenuation. As a result, the switching transistor as well as the other power devices, such as for example the transformer and the circuit elements connected to the secondary winding of this transformer are subjected to a substantial thermal load. A current limiter connected, for example, to the secondary winding of the transformer could then be a remedy but would require an undesirably large number of component parts. This number increases as the voltage to be supplied by the power supply device is selected higher.